1. Field of the Invention
The present invention relates to an improvement on a crystal holding apparatus for holding a corrugated portion of a crystal in a pulling apparatus which pulls the monocrystal according to, for example, the Czochralski method (CZ method).
2. Description of the Related Art
The CZ method has conventionally been used in the manufacture of an ingot of a semiconductor material such as silicon. In the CZ method, a seed crystal held by a seed chuck is brought into contact with the surface of material melt contained in a crucible, and is then pulled while being rotated. At this time, the pulling speed is controlled such that a neck portion is first formed below the seed crystal, and a straight cylindrical monocrystal portion (i.e., ingot) having a large diameter is then formed. The formation of the neck portion permits elimination of dislocations from the straight cylindrical portion, thereby improving the quality of the ingot. In recent years, the weight of a monocrystalline ingot to be pulled has tended to increase due to an increase in the diameter of the monocrystalline ingot and an improvement in manufacturing efficiency, and consequently the strength of the neck has tended to be insufficient.
In view of the foregoing, there has been adopted a technique in which a stepped engagement portion consisting of an increased-diameter portion and a reduced-diameter portion is formed between the neck portion and the straight cylindrical portion of a monocrystal, and the monocrystal is pulled while a lifting jig holds the stepped engagement portion. Examples of such a technique are described in, e.g., Japanese Patent Application Laid-open (kokai) Nos. 62-288191, 63-252991, 63-285893, and 3-295893. For instance, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 3-285893, a stepped engagement portion is formed while a seed crystal is pulled, and when the stepped engagement portion reaches a position corresponding to gripping levers of a lifting jig disposed at a predetermined height, the gripping levers grip and pull the stepped engagement portion, so that the weight of the monocrystal, which until this point has been borne by a pulling wire, is shifted to the lifting jig.
More recently, safer and more practical pulling apparatuses have been developed. In a pulling apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-2893, a measure is employed to prevent a lifting jig--which holds the stepped engagement portion of a monocrystal--from opening easily, even when a large load acts on the lifting jig, to disperse stresses acting on holding portions that are in contact with the stepped engagement portion, and to prevent contamination of material melt, which contamination would otherwise occur due to particles generated at a sliding portion of the lifting jig or at other portions.
However, the above-described conventional crystal holding apparatus has an unsolved problem as follows. That is, the holding portions of the lifting jig--which portions are actually brought into contact with the surface of the stepped engagement portion of a monocrystal and holds it--has been made of a material having high heat resistance and hardness, such as stainless steel or molybdenum. However, recent studies reveal that when a hard material is used, each of the holding portions contacts a monocrystal via a single point, and therefore stress concentration occurs, with the result that the monocrystal breaks from the contact point even if the applied load is relatively small (approximately 180 kgf). Therefore, if the crystal holding apparatus were used to pull a large monocrystal having a mass of over 180 kg, the monocrystal would fall down during the pulling operation. Even when a monocrystal having a mass of about 100 kg is pulled, there is a large possibility that the monocrystal would fall during pulling operation in consideration of the design safety factor and the brittleness of silicon. Therefore, there has been a strong demand for a measure to solve the above-described problem.